...is a bitch. Gone are the carefree days when my most passive-aggressive ambition was to get my mother to buy me an ice cream cone. My younger days were not totally without stress and misery: homework, exams, competition, peer pressure, trying to win a medal or a teacher's affection. However, at least the summers were spent completely relaxed and carefree out in the ocean, without the desire for fame and fortune or a life-time acheivement award, that your whole self-worth would be based on how someone liked or disliked you because of your work, not who you were.
Ambition does motivate you. It helps to push you where you want to be -- a better person, a better life. It doesn't necessarily give you happiness, but at least it gives you a false sense of security. The flip side of the coin is of course insecurity. Ambition usually comes with self-doubt, the evil twin that won't go away. Peace goes out and anxiety marches in. The worst part is, sometimes when you do get to where you want to be, you're not happy anyway. You look up, and you see another pedestal. You try to leap again, only realizing that the pedestal you're standing on is not that secure anyway.
Ambition may be a spring to launch you forward, but if there is not a safety net, you will end up splattered on the ground, someday, somehow.